Compact multimode laser with rapid wavelength scanning

ABSTRACT

In accordance with the present invention, a compact laser system with nearly continuous wavelength scanning is presented. In some embodiments, the compact laser system can be scanned over a broad range. In some embodiments, the compact laser system can be scanned at high scan rates. In some embodiments, the compact laser system can have a variable coherence length. In particular, embodiments with wavelength scanning over 140 nm with continuously variable scan rates of up to about 1 nm/μs, and discrete increase in scan rates up to about 10 nm/μs, and variable coherence lengths of from 1 mm to about 30 mm can be achieved.

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 60/647,078, “Compact Laser with Continuous Wavelength Scanning,” byAlex Cable, Michael Larsson, Lars Sandstrom, and Bengt Kleman, filed onJan. 24, 2005, which is herein incorporated by reference in itsentirety.

BACKGROUND

1. Field of the Invention

The present invention is related to a tunable multimode laser and, inparticular, to a compact laser with mode-hopping wavelength scanning.

2. Discussion of Related Art

External Cavity Lasers (ECLs) are now commonly used in severalapplications where a continuously tunable laser source is needed.Although there are many commercial manufacturers of ECLs, covering abroad range of designs on the market, there are still many applicationsthat would benefit from a dramatic increase in the tuning speed thatcurrent designs cannot offer, even if this increase comes at the expenseof other operating parameters of the system such as the dynamiccoherence length and power stability.

More specifically, fast ECLs with tuning speeds greater than 1,000,000nm/s, tuning ranges of at least 5% of the center wavelength, andcoherence lengths of at least several millimeters are not currentlyavailable commercially. Lack of wavelength tuning speed is a majordisadvantage in present ECLs in that there are many applications thatwould benefit from substantially increased tuning speeds. Current ECLs,for example, may have a sufficient tuning range and also a trulysingle-mode behavior, and thus a coherence length of significantly morethan a few millimeters (often many meters), but their tuning speed islimited to approximately 1,000 nm/s or less.

Note that there are tunable lasers that offer rapid, highly precisewavelength switching. For example, Syntune of Kista Sweden offers onesuch switchable laser that is capable of point-to-point wavelengthswitching times of less than 50 ns. However, such systems do not offercontinuous sweeping of their output wavelength. Instead, they movediscretely from one wavelength to another without generating awell-defined wavelength during the time interval required to move fromone stable wavelength to another. Further, the Syntune laser requires asignificant manufacturing infra-structure and is not easily implementedat new wavelengths. Another product that has some of the desiredcharacteristics is offered by Micron Optics. The Micron Optics system isreferenced in a scientific article that appeared in Optics Express, 8Sep. 2003, Vol. 11, No 18, pages 2183 to 2189. The Micron Optics sourceprovided 2 mW of optical power, a sweep time of 3.5 ms, a centerwavelength of 1308 nm, and a FWHM (full width half maximum) sweepbandwidth of 87 nm. The Micron Optics system, assuming that the totalswept range (defined as the 99% power points) is approximately twice theFWHM sweep bandwidth of 87 nm, results in a sweep speed of approximately50,000 nm/s. The Micron Optics system, however, utilizes an intra-cavityfiber optic element and is susceptible to unwanted polarizationvariations as well as being limited to tuning speeds much less than thatrequired by high-speed applications. Additionally, the Micron Opticssystem is a fiber based laser and as such it is difficult to make thecavity sufficiently short to enable very high tuning rates. The lengthof a tunable laser determines the transit time of a photon or a group ofphotons. If a group of photons of a specific wavelength takes too longin transiting the laser, when they return to the filter element thefilter element could be tuned to another wavelength and hence provideunwanted attenuation of the laser action. The article by R. Huber et.al. in Optics Express 2 May 2005 V13, No 9 page 3513 to 3528 provides amore complete description of this limitation.

The typical design solution for an ECL is to provide a mirror or gratingrotatable around a pivot axis that can be rotated to hold the laser inthe same longitudinal mode over the laser's entire tuning range. Thepivot axis is chosen such that both equation 1 and equation 2, shownbelow, can be simultaneously satisfied:λ_(N) =d(sin α+sin β),  (1)Nλ _(N)=2L.  (2)In Equations 1 and 2, λ_(N) is the average instantaneous outputwavelength of the laser, N is the longitudinal mode number of the laser,d is the grating constant measured in the same units as the wavelength,α is the angle of incidence of the light field upon the grating, β isthe angle of diffraction of the light field off the grating, and L isthe optical path length of the laser cavity. If the two conditions ofEquations 1 and 2 are simultaneously fulfilled during the wavelengthtuning with N fixed, the resulting laser will tune continuously andwithout any longitudinal mode hops. While there are several mechanicalsolutions that may satisfy the requirements of the two equations shownabove, those that are found commercially or in the scientific literatureare limited, due to their size, to tuning speeds significantly below thedesired tuning speeds. The mechanics required to rotate a mirror orgrating in accordance with the Equation 2 above with N fixed are almostalways complicated mechanisms that have large inertial mass, whichprevents them from being actuated at the high speeds required for rapidtuning.

To our knowledge, the fastest commercially available continuously sweptsingle longitudinal mode tunable laser is offered by New Focus and isadvertised as providing 1000 nm/s scan speeds. In order to reach thedesired scan speeds of 1,000,000 nm/s, the moving optical element, whichis typically a mirror or optical grating, is best rotated around itscenter of mass and is best kept as small as possible. Utilizing aresonant scanner provides a convenient means to achieve both theseobjectives; however this method typically has the rotation axis centeredon the moving optical element, and Equation 2 cannot generally be metwhen N is held constant. In other words, the longitudinal mode conditionNλ_(N)=2L cannot be satisfied continuously, and the laser radiation willjump from one longitudinal mode to another when the tuning range issignificant enough to be of interest.

The mode jump can be one or several mode distances wide, the modedistance being:Δλ=λ²/2L.With these mode jumps present, the laser will no longer be acontinuously tunable laser, and the dynamic coherence length of thelaser will become erratic and degrade below the static coherence lengthof the system. In order to reduce the size of the mode jump, the cavitylength can be increased, but that will also make the laser more disposedto multimode behavior and thus also contribute to a short coherencelength.

In U.S. Pat. No. 5,956,355 (the '355 patent), a laser design in whichthe length of the cavity of a widely tunable single mode laser isadjusted to compensate for changes in wavelength is disclosed. The '355patent disclosed that the laser could be made to provide nearlycontinuous frequency tuning through an appropriate choice of the lasercavity components and geometry while also offering a high scan rate. Themethod proposed in the '355 patent used a steerable mirror and adiffraction grating-which provided the wavelength selectivity-orientedsuch that the steerable mirror sweeps the light field across the gratingin such a way to have the cavity length change to offset the changes inwavelength with the goal of maintaining a near balance in Equation 2above, with N held constant. It was further proposed in the '355 patentthat an additional element could be added such that the residual erroror imbalance in Equation 2 could be accounted for such that a precisebalance of Equation 2 could be maintained across a broad tuning range.While this approach seems feasible, to our knowledge, there has been nosuccessful implementation of this proposed design.

The instantaneous coherence length of a wavelength swept laser can bemeasured utilizing one of several methods, usually involving differentkinds of interferometers. One such method is to utilize a fiber basedMichelson interferometer. The coherence length, L_(c), is given by:L _(c)=2×HWHM,were HWHM is the displacement of one of the mirrors in theinterferometer required to change the interferogram from 100% of maximumto 50% of maximum. Note that the factor of 2 accounts for the doublepass (forward and backward) in the moving arm of the Michelsoninterferometer.

There is a need in many applications to simultaneously reach high nearlycontinuous-tuning speeds and dynamic coherence lengths of at least a fewmillimeters. This is the case in Swept Source Optical CoherenceTomography (SS-OCT) as well as in some optical remote fiber sensing andoptical component testing applications. In SS-OCT the coherence lengthof the source sets a limit on the imaging depth, with the potentialimaging depth scaling linearly with the coherence length.

SUMMARY

In accordance with the present invention, a nearly continuous wavelengthscanning multimode laser is presented that has a large scanning ratewhile maintaining sufficient coherence length to be utilized inswept-source applications. In some embodiments of the present invention,a compact laser light source can have a wavelength scanning range ofover about 140 nm with variable scan rates of up to about 10 nm/ps and acoherence length of between about 3 mm to about 30 mm.

Instead of trying to maintain a single longitudinal mode or nearlysingle longitudinal mode as the laser is scanned in wavelength, as isattempted in other systems as described above, some embodiments of thepresent invention provide a laser design that is intentionallymultimode. Additionally the tuning mechanism according to someembodiments of the present invention is such that as the laser is tunedin wavelength the family of longitudinal cavity modes changessignificantly, by sometimes many tens of thousands of modes.

A compact laser system according to some embodiments of the presentinvention includes an optical cavity, a light generating and opticalgain medium section, and a beam shaping optical system; an opticalpropagation medium; and a fast mechanical wavelength tuning sectionpositioned to receive light from the light generating section and theoptical propagation medium, the fast mechanical wavelength tuningsection-including a dispersive part-to retro reflect selectedwavelengths of light to the optical propagation medium, and the lightgenerating section, wherein the wavelength of the laser system can bescanned. In some embodiments the first end-reflector of the opticalcavity is created from one end of the light generating and optical gainmedium. In some embodiments the second end-reflector of the opticalcavity is embedded within the fast mechanical wavelength tuning section.In some embodiments the second end-reflector of the optical cavity is aseparate optical element. In some embodiments, the fast mechanicalwavelength tuning section includes a dispersive part, a spectral opticalfiltering part, and a reflector part. In some embodiments, the reflectorpart is the dispersive part. In some embodiments, the reflector partserves as a second end-reflector of the laser system cavity. In someembodiments, the reflector part is combined with the spectral opticalfiltering part. In some embodiments, the total optical path length ofthe cavity is as short as 25 mm. In some embodiments the total opticalpath length of the cavity can be varied from 25 mm to many meters.

These and other embodiments are more fully discussed below with respectto the following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of laser system according to thepresent invention.

FIG. 2 illustrates another embodiment of laser system according to thepresent invention.

FIG. 3 illustrates another embodiment of laser system according to thepresent invention.

FIG. 4 illustrates another embodiment of laser system according to thepresent invention.

FIG. 5 illustrates another embodiment of laser system according to thepresent invention.

FIG. 6 illustrates the coherence length versus scan speed for someembodiments of laser system according to the present invention.

FIG. 7 illustrates another embodiment of laser system according to thepresent invention.

FIG. 8 illustrates a schematic representation of a light sourceaccording to the present invention.

FIG. 9 shows a filter function for the cavity of a light source such asthat illustrated in FIG. 8.

FIG. 10 illustrates the output power of an embodiment of a light sourcesuch as that illustrated in FIG. 8 as a function of drive current.

FIG. 11 is an OSA trace of the ASE spectrum of the gain element with theinternal cavity blocked to prevent lasing for an embodiment of a lightsource such as that illustrated in FIG. 8.

FIG. 12 illustrates an ASE spectrum of an embodiment of light sourcesuch as that illustrated in FIG. 8.

Components and other details of one and the same type have a consistentnumbering in the different figures. The first digit in the number refersto the numbering of the figure and the following digits refer to thespecific type of component or detail.

DETAILED DESCRIPTION

In accordance with some embodiments of the present invention, a tunablelaser system that provides more than 5 mW of average optical powerdelivered from a single mode optical fiber, a dynamic coherence lengthof greater than about 2 mm, a tuning range (measured at the 99% powerpoints) of approximately 5% of the center wavelength when operating at acenter wavelength of, for example, about 850 nm, and approximately 10%of the center wavelength when operating at a center wavelength of, forexample, about 1330 nm, and a tuning speed that is continuously variablefrom 0 to about 2,000,000 nm/s is presented. In some embodiments,additional discrete increases in tuning speed up to about 10,000,000nm/s can also be obtained with the upper limit on the tuning speed beinglimited by the availability of appropriate high speed scanners or highspeed tunable optical filters. In some embodiments, the polarizationstate of laser light emitted from the laser is linearly polarized andhighly stable. Once coupled into a single mode fiber, the polarizationstate can easily be controlled via a manual polarization controller. Theintensity profile as a function of wavelength can have a shape that isvery roughly Gaussian and can be further shaped to better follow aGaussian profile by electrically controlling the drive current appliedto, for example, a semiconductor-based gain element. Examples ofapplications that would benefit from the improved performance offered byrapidly scanned tunable lasers such as is provided by some embodimentsof the present invention include metrology, spectroscopy, medicalimaging, and any other optical technique that requires a source ofrapidly tuned laser light with coherence lengths in the range of about 2mm to about 50 mm.

FIG. 1 shows a light source 100 according to some embodiments of thepresent invention. As shown in FIG. 1, a light source according to thepresent invention includes a light generating section 101, atransmission section 102, and a tuning element section 103. In someembodiments of the present invention, light generating section 101 caninclude a gain element with the intra-cavity side having a curvedwaveguide and an anti-reflective (AR) coated outer surface or facetwhich suppresses self-lasing and allows the element to serve as aneffective gain medium for an external cavity laser. And the other facetof the gain element can include a partially reflecting surface, forexample with a reflectivity of approximately 20%, which serves as theoutput coupler of the laser. In some embodiments of the invention, bothsides of the gain element can be AR coated and the mirror defining oneside of a laser cavity can be positioned appropriately relative to thegain element. In some embodiments, the output coupler reflectivity canrange from approximately 10% to 50%.

Light-generating section 101, in some embodiments, can be of theconventional type for extended cavity laser systems. Such systemstypically include a first optical cavity end-reflector that is often,but not always, one of the outer surfaces of a broadband gain element.The other surface of the gain element emits a divergent light field thatis collimated with a short focal length lens. Light generation section101 may be referred to herein as the light-generation section or assimply “the first section” and may contain any number of gain elementsto achieve lasing at various wavelengths. The active laser gain mediumcan be made, for example, from semiconductors, e.g. gallium arsenide(GaAs), indium gallium arsenide (InGaAs), gallium nitride (GaN), indiumphosphide/indium gallium arsenide phosphide (InP/InGaAsP), and indiumphosphide/indium gallium arsenide/indium gallium arsenide phosphide(InP/InGaAs/InGaAsP); certain glasses (e.g. silicate or phosphateglasses), including both bulk glasses and optical fibers, typicallydoped with some rare-earth ions (e.g. of neodymium, ytterbium, orerbium) or transition metal ions (e.g. of titanium or chromium); certainoptical crystals, also doped with some laser-active ions; gases, e.g.mixtures of helium and neon, nitrogen, argon, carbon monoxide, carbondioxide, or metal vapors; or liquid solutions of certain dyes.

The beam shaping optics of light generation section 101 can be chosenfrom a broad selection of optical elements commonly used to shape orcollimate the output of a laser diode. The collimation lens, which canbe, for example, a GRIN lens or an aspheric lens, are used with thefocal length of the optic chosen to provide full illumination of theoptical grating when that is the element chosen to provide thewavelength dispersion. If the first optical cavity end-reflector is notattached to the gain element, but is a separate optical element, then anadditional collimation lens could be used to collimate the light emittedfrom the first surface of the gain element. The beam shaping optics canbe, for example, an anamorphic prism pair or cylindrical lens.

Embodiments of light sources according to the present invention arearranged to form multimode lasers as opposed to the conventional singlemode operations. A multimode laser is a laser having multiplelongitudinal modes (i.e., a number of discrete wavelengths that inaggregate make up the lineshape of the laser). In general, changing thelength of the cavity, the filter function of the cavity, the gain, theloss, the alignment, or the reflectivity properties of the end-mirrorsof a laser cavity changes the number of modes that can lase within thecavity (the last three of these change the number of modes by changingthe lasing threshold). As used in the present invention, the term “gainmedium” can be any element that provides optical gain, includingsemiconductor elements, rare-earth doped optical fiber based amplifiers,organic dyes, or other material. The terms “wavelength swept” wavelengthtuned,” “frequency tuned,” “frequency swept,” and “wavelength agile” aregenerally understood to be interchangeable. When referred to, the“cavity filter function” is the effective filter function of the entirelaser cavity after it has been assembled, measured by measuring thespectral properties of the light that is emitted from the system withthe light source operating below the lasing threshold. The terms“optical source” and “laser system” or “laser” are herein utilizedinterchangeably.

In general, a light source according to the present invention includes alaser built from two cavity end-mirrors, a gain element, and a tunablewavelength selecting device. These components are brought together intoa configuration in which both the cavity length (which sets the modespacing) and the filter function of the cavity are predetermined suchthat the laser provides the desired output characteristics. The maincomponents also form a multimode laser; the task of correctly specifyingeach of the elements to achieve the unique characteristics of a lightsource with a coherence length greater than about 2 mm while providingfor sweep speeds of up to about 10,000,000 nm/s over a wavelength rangethat is approximately 5% of the center wavelength of the laser isexemplified below.

As shown in FIG. 1, in some embodiments of the present invention lightgenerating section 101 can be followed by transmission section 102,which can be a free space section. Alternatively, transmission section102 can contain a specific propagation medium, for example an opticalfiber. One advantage of using an optical fiber is to allow light sourcesaccording to the present invention to be further compressed in size aswell as to provide additional wavelength selectivity within the cavityof the light source such as source 100. In some embodiments, the lengthof transmission section 102 can be adjusted from just a few millimetersto many meters to adjust the mode spacing of the laser light source 100as well as the average time a photon spends in the cavity of lightsource 100.

Light propagation section (or second section) 102 can either be freespace or a medium such as a single mode or single mode polarizationpreserving optical fiber. Second section 102 can form the major part ofthe optical path within the optical cavity of the light source. In someembodiments, the length of second section 102 can be adjusted tooptimize the performance of the laser system of the light sourceaccording to the present invention. As the length is changed, thelongitudinal mode distance in the optical cavity as well as thecoherence properties of the light source change. For each embodiment oflight source, the length of the cavity can be adjusted until the laserperformance is optimized in terms of coherence length, tuning range, andoptical power. The performance of laser cavities with various lengthsettings for the second section have been explored experimentally; theresults indicate that acceptable performance can be obtain over a verybroad range of distances. Embodiments of the light source with shortcavity lengths, e.g. <50 mm, good overall performance can be obtainedwhen the external cavity is well aligned to provide efficient couplingof the filtered light back into the gain element of first section 101.This efficient coupling ensures that the residual reflections from theintra-cavity side of the gain element do not cause an unwanted amplitudemodulation of the output power as a function of wavelength. Pooralignment or a significant residual reflection from the intra-cavityfacet or facets of the gain element can lead to amplitude modulations asa function of wavelength in the output, which have a frequency given bythe free spectral range of the gain element. At longer cavity lengths,e.g. >400 mm, the performance of the light source can still remainacceptable. However, as the length of the cavity is increasedsignificantly beyond this level the scan rate of the resulting lightsource can become limited. This effect is attributed to the fact thatthe cavity needs sufficient time to allow each wavelength to realize asufficient number of passes around the cavity in order to achievesufficient gain to sustain an appreciable output power. Hence, thetuning speed of the optical source has an upper limit that is related tothe cavity length, the gain characteristics of the gain element, and thelosses in the external cavity optics including the coupling losses backinto the gain element.

A fast wavelength-tuning section 103 receives light from thelight-generating section 101 that is passed through transportationsection 102. This fast wavelength-tuning section 103 spectrally filtersand retro-reflects selected wavelengths of light to light generatingsection 101. Tuning section 103 can include, in addition to a wavelengthselective component, a spectral-optical-filtering-and-reflector device.The reflector part of the spectral-optical-and-filtering device canserve as a second optical end-reflector of the cavity of the lasersystem.

Tuning section (or third section) 103 includes a tuning element orelements and a supplemental spectral filtering element or elements whichreceives light from transportation section 102. Third section 103 actsto provide both the enhanced spectral filtering of the cavity lightwhile also providing the tuning system for the laser. Third section 103also provides a high efficiency retro-reflection required to form thelaser resonator of the light source. Third section 103 provides theselected wavelengths of the intra-cavity light field that are returnedto the first section of the system for amplification.

Some specific embodiments of light sources according to the presentinvention are discussed below with discussion of individual componentsof light generating section 101, transport section 102, and tuningsection 103. The performance of some of the embodiments have beenstudied experimentally, and the anticipated scanning speed and tuning ata high frequency repetition rate, i.e., number of scans per secondcovering the relevant wavelength range independent of scan direction, upto about 16 kHz, with a coherence length of 11 mm or more has beendemonstrated. Further, at slower tuning speeds the coherence lengthincreased.

Some embodiments of light source according to the present invention forma tunable external cavity laser system with an output power in themilliwatt range, a coherence length of a couple of millimeters to acouple of tens of millimeters, and an upper tuning speed that wouldenable video rate acquisition of images in a SS-OCT system. Theusefulness of various embodiments of the invention, however, is notlimited to tomographic applications, and laser system according to someembodiments of the invention may also be of interest in othermeasurement applications that require high speed tunability of acoherent source of optical radiation.

Some embodiments of the present invention utilize a moderate lengthcavity, with a round trip cavity length of approximately 0.1 meter toapproximately 1 meter, along with a supplemental spectral filter thatnarrows the filter function of the cavity while also acting as a highefficiency retro-reflector. The cavity length of the light source isadjusted for a given spectral filter such that the desired performancecharacteristics of the laser are optimized; these could be coherencelength, optical output power, optical output intensity noise, as well asthe size and weight of the light source. In general the narrower thepass band of the spectral filter, the longer the coherence length;however, the desire to obtain a high output power from the lasertypically provides a lower limit on the pass band of the supplementalspectral filter. The additional spectral filter is referred to as asupplemental filter because in most commercially available ECL designsthe aperture of the gain element along with the optical gratingtypically found in the laser cavity both act to form the primaryspectral filtering element. The present invention achieves tuning speedsof from about 1000 nm/s to about 10,000,000 nm/s with the coherencelength decreasing as the tuning speed is increased for a specific lasercavity configuration. When operating at approximately 2,000,000 nm/s acoherence length from an example light source according to the presentinvention of approximately 12 mm was obtained. These measurements werefor a system operating with a center wavelength of 1.33 μm and a tuningrange of approximately 130 nm. FIG. 6 shows the coherence length as afunction of tuning speed for a system with a fixed filter function but acontinuously variable scan speed.

One embodiment of the present invention was tested within an SS-OCTimaging system; the design of this particular laser is detailed in thefollowing paragraphs. Details on the results of the OCT imaging studyhave been published in R. Huber et al. Optics Express 26 Dec. 2005 V13,No 26 page 10523 to 10538 (“Huber et al.”), which is herein incorporatedby reference in its entirety.

FIG. 8 shows a schematic representation of a light source 800 accordingto the present invention. An embodiment of light source 800 was testedand several test results are provided below. Further, a system designedas in light source 800 was utilized to perform the measurements includedin Huber, et al. As shown in FIG. 8, light generation section 101includes gain element 811 and aspheric lens 812. Gain element 811 can beapproximately 1 mm in length with an estimated index of refraction ofabout 3.5. In some embodiments, gain element 811 can be formed ofInP/InGaAsP semiconductor optical amplifier. The left facet of gainelement 811 serves as the output coupler for light source 800; the leftfacet (or output facet) reflectivity of gain element 811 in someembodiments is estimated to be in the range of about 15% to about 20%.As discussed above, in some embodiments of light source according to thepresent invention, the left facet of gain element 811 can be ananti-reflection (AR) coated and coupled, potentially through otheroptics, to a reflector. The reflector may be an output coupler but insome embodiments may form the substantially reflecting end of theresulting laser cavity.

Gain element 811 can, in some embodiments, be bonded to athermo-electric (TE) cooler to maintain a constant temperature. In someembodiments, gain element 811 can be maintained at a temperature ofapproximately 22° C.

The intra-cavity side of gain element 811 can utilize a curved waveguideand an AR coated facet. In some embodiments, an estimated effectiveintra-cavity facet reflectivity of approximately 10⁻⁴ is achieved, whichsuppresses self-lasing and allows element 811 to serve as a moreeffective gain medium for the external cavity of light source 800. Lightfrom gain element 811 is coupled into aspheric lens 812. In someembodiments, aspheric lens 812 can have a 2 mm focal length and is ARcoated. In some embodiments, both optical surfaces of aspheric lens 812can be convex.

In light source 800, transportation section 102 is a free space region.In some embodiments, the free space region of transportation section 102can be approximately 370 mm. The free space region of transportationsection 102 can be utilized to adjust the overall length of the cavityformed in light source 800.

Light from the free space region of transportation section 102 iscoupled into tuning section 103 of light source 800. Tuning section 103includes grating 813, lens system 814, slit assembly 816, and mirror815. In some embodiments, diffraction grating 813 can have a rulingdensity of about 1017 lines/mm. Diffraction grating 813 can be mountedon a resonant scanner 817 that provides an angular displacement and aparticular operation frequency. For example, a resonant scanner 817 thatprovides a total angular displacement of approximately 14 degrees at 8kHz can be obtained from Electro-Optical Products Corp.

Light from grating 813 can be coupled into lens system 814. In someembodiments, lens system 814 can be an achromatic doublet lens. In someembodiments, lens system 814 can be optimally arranged for operation atmultiple wavelengths. For example, in some embodiments three wavelengthswere used in the optimization of lens system 814: 1.0 μm, 1.3 μm, and1.5 μm. In some embodiments, lens system 814 can have a 45 mm focallength.

Lens system 814 focuses light from scanner 813 onto slit 816. In someembodiments, slit 816 can be a 10 μm slit that is bonded directly ontothe reflective surface of a broadband dielectric mirror 815. In someembodiments, mirror 815 can have a reflectivity of greater than about98.5% over the complete operating range of light source 800 and isplaced at the back focal plane of lens system 814. As such, thecombination of mirror 815 and slit 816 form the back-reflector of alaser resonator of light source 800.

In some embodiments, light source 800 can be coupled into a collimatinglens system (not shown). The collimating lens system may, for example,be an AR, coated aspheric lens, for example with a focal length of 0.7mm. In some embodiments, light from the collimating lens system may becoupled into an isolator, for example a −55 dB optical isolator, thatprevents back-reflections from re-entering the laser cavity of lightsource 800. In some embodiments, light from the isolator can be coupledthrough an aspheric lens into an optical fiber. For example, an ARcoated, 4 mm focal length aspheric lens can be utilized to couple lightinto an AR coated single mode fiber.

Light source 800 can be wavelength sweep by rotating grating 813 withresonant scanner 817 as shown in FIG. 8. Resonant scanner 817 rotatesgrating 813 to sweep the various wavelengths across slit 816, therebymaintaining a constant cavity length as the laser is tuned. The laserdynamics of light system 800 can be understood by considering thesituation where a single longitudinal mode is lasing within the cavityformed between mirror 815 and the partially reflective surface of gainelement 811. If the rotation axis of grating 813 is perpendicular to theplane defined by the optical light field of light source 800 and thelight field is centered about the rotation axis, the cavity length willbe kept substantially constant when grating 813 is rotated. Therefore,there is no wavelength change within the cavity for very small rotationsof grating 813. Once the rotation is sufficient to ensure that the nextlongitudinal mode has lower loss, the laser formed in light source 800will hop to this next mode, and the center frequency of the laser isconsequently shifted by one or a few longitudinal cavity modes which isgiven by the free spectral range of the cavity. In some embodiments, thefree spectral range of the laser cavity is about 330 MHz. Furtherrotation of grating 813 leads to this pattern repeating itself, wherebythe laser wavelength (i.e., the output frequency of light source 800) asa function of grating angle follows a “staircase” shaped tuning curve.In reality, the finite width of the filter function of grating 813 canallow many longitudinal modes, approximately 80 for some embodiments oflight source 800, to lase simultaneously. Therefore, the laser generatesa comb of frequencies that tune in a stepwise fashion with the step sizebeing equal to the longitudinal mode spacing.

As shown in L. A. Kranendonk, R. J. Bartula, and S. T. Sanders,“Modeless operation of a wavelength-agile laser by high-speed cavitylength changes,” Opt. Express 13, 1498-1507 (2005), varying the cavitylength at high speeds produces operation that is similar to theoperation of a frequency shifted feedback laser, which is discussed inP. I. Richter and T. W. Hansch, “Diode-Lasers in External Cavities withFrequency-Shifted Feedback,” Opt. Commun. 85, 414-418 (1991). A value Ris defined as the relative frequency change of the cavity modes withrespect to their free spectral range during one optical roundtrip. Thelaser in the Kranendonk reference tunes in a modeless regime with avalue R>>0.05 using a rapid change in cavity length that isintentionally introduced in order to achieve this modeless operation. Incontrast, light source 800 and some other light sources according to thepresent invention tune with distinct cavity modes.

In light source 800 shown in FIG. 8, there is substantially no change inthe cavity length while the laser is being tuned. As a result, the valueR is substantially 0. This arrangement, therefore, provides an optimumbuild-up time for a mode structure, which has been demonstrated toprovide a higher output power stability.

A number of other cavity designs were explored with non-zero values ofR, and performance results were compared to that of an embodiment oflight source 800 shown in FIG. 8. These designs had similar layouts withthe same sequence of components, however with varying cavity lengths.The non-zero values of R were obtained by offsetting the optical axisfrom the axis of rotation of the diffraction grating. In each instance,the instantaneous coherence length was found to be shorter than for anembodiment of light source 800 such as that shown in FIG. 8.

To estimate the coherence properties of embodiments of light source 800while it was being frequency tuned, the fringe contrast was measuredwhile the output was coupled into a Michelson interferometer. Theamplitude of the fringe signal was measured as a function of theinterferometer arm length difference. A 3 dB drop over about 3.5 to 4 mmarm length difference in the Michelson interferometer was observed. Thecoherence length for the static case when the laser is not frequencyscanned is determined by measuring the spectral linewidth of the outputwith an optical spectrum analyzer. A resolution limited static linewidthof <0.02 nm was measured, which corresponds to a coherence length ofgreater than about 8 cm.

The filter function of a cavity of light source 800 is taken with theinjection current set to 45 mA, which is well below threshold for gainelement 811 formed from an InP/InGaAsP semiconductor optical amplifier.In some embodiments, care was taken to ensure that the FWHM of theresulting lineshape did not change appreciably as the drive current wasincreased from the point at which the lineshape just started to emergefrom the noise.

As shown in FIG. 8, utilizing a 10 μm slit for slit 816 limits the FWHMpass band of the cavity to approximately 0.17 nm. FIG. 8 was obtainedwith the location of the grating fixed in its center position. Care wastaken to ensure that the FWHM of the filter function did not changeappreciably as the current was increased from below threshold to the 45mA used for this measurement. The laser threshold current was measuredto be approximately 62.5 mA, as is shown in FIG. 10. FIG. 10 illustratesthe output power of an embodiment of light source 800 as a function ofdrive current. FIG. 10 was obtained using a calibrated optical powermeter. The threshold occurs at approximately 62.5 mA of injectioncurrent at a temperature of 22° C. The optical spectrum analyzer (OSAmodel Anritsu MS9710A) was set to its highest resolution of 0.07 nm anda VBW (video bandwidth) of 1 kHz was used for this measurement. As thecurrent was increased beyond 62.5 mA, with the scanner still in the offposition, the FWHM of the spectral profile narrowed quickly beyond themeasurement resolution of the OSA. As expected the static linewidth ofthe laser is substantially smaller that the dynamic linewidth.

FIG. 11 is an OSA trace of the ASE spectrum of the gain element with theinternal cavity blocked to prevent lasing for an embodiment of lightsource 800 as discussed above. The injection current to the gain elementwas set to 300 mA, and the TE cooler that controls the temperature ofthe gain element was set for 22° C. The OSA resolution was set to 0.07nm and 5,000 data points were acquired. The structure on the right sideof the plot is assumed to be due to water vapor; both these absorptionfeatures as well as the intensity modulation that is apparent in thisplot are discussed below.

The strong absorption lines apparent in FIG. 11 were confirmed to be aresult of water vapor with the strongest lines occurring between 1340and 1410 nm. See reference M. P. Arroyo, R. K. Hanson, “Absorptionmeasurements of water-vapor concentration, temperature, and line-shapeparameters using a tunable InGaAsP diode laser,” App. Optics, V 32, No30, (1993) pg 6104-6116. The effect of intra-cavity water vaporabsorption losses on the quality of the OCT imaging capabilities of thesystem were investigated. The system was purged overnight with drynitrogen. OCT images were taken before and after purging; when comparedno discernable effects were noted. It is, however, anticipated thatembodiments of light source 800 intended for wide scale use would needto be hermetically sealed in an inert atmosphere, as is the case formany commercially produced external cavity laser diode systems.

The fast intensity modulation apparent in FIG. 11 were found tocorrelate with the Fabry Perot modes of gain element 811 that resultfrom the residual reflectivity of the intra-cavity facet. Thismodulation of the ASE spectrum leads to an amplitude ripple in thespectrum of the output of the frequency swept laser. Additionally, thisartifact leads to unwanted anomalies in the OCT images. Both the qualityof the AR coating on the intra-cavity facet and the total efficiency ofthe external cavity were found to significantly effect the magnitude ofthe ripple.

In order to ensure stable operation of these embodiments of light source800 over a broad tuning range, as well as to fulfill the need to reducethe spectral ripple described above, strong feedback from the externalcavity can be utilized. Strong feedback may be defined as the conditionwhere R_(EC)>>R_(SG) where R_(SG) is the reflectivity of theintra-cavity facet of gain medium 811, and REC is the effectivereflectivity of the extended cavity when all optical elements, includingthe coupling losses of the feedback back into gain medium 811, areconsidered. See reference A. Olsson, C. L. Tang “Coherent OpticalInterference Effects in External-Cavity Semiconductor Lasers,” IEEE J.of Quantum Electronics QE-17 No 8, (1981) pp 1320. Therefore R_(EC) isdependent upon the reflectivity of each of the optical surfaces withinlight source 800, as well as the spatial mode structure and alignmenterrors of the light field within the cavity.

Light source 800, as shown in FIG. 8, partially ensures that REC isrelatively independent of the wavelength across the emission wavelengthrange of gain element 811 and also ensures that cavity length changesdid not adversely effect the performance of light source 800. Lightsource 800 includes a lens 814 and mirror 815 combination that istypically referred to as a “cat's-eye” configuration, where mirror 815is placed in the focal plane of lens system 814. The optical arrangementof light source 800 illustrated in FIG. 8 can provide a reducedsensitivity of the feedback to angular misalignments of theretro-reflector while also ensuring good optical performance over theentire ASE spectrum of gain element 811. See reference J. J. Snyder“Paraxial ray analysis of a cat's-eye retro-reflector,” Applied Optics V14, No 8, (1975) pp. 1825. FIG. 12 illustrates an ASE spectrum of anembodiment of light source 800 and was produced using the same AnritsuOSA as mentioned above with the peak hold feature enabled and a 20 mssample interval. The OSA resolution bandwidth was set to 1 nm, and 500data points were acquired using a 20 ms peak hold feature of the Anritsumodel MS9710A OSA. The full width at half-maximum was determined to beapproximately 122 mn with an average output power of 15 mW.

The OSA peak hold feature was utilized to capture and average a largenumber of forward and backward scans from light source 800, which wasoperating at 16 kHz due to the inherently low temporal response of thespectrum analyzer. It should be noted that the spectral data acquired bythe OSA will be distorted by the sinusoidal nature of the resonantscanner based tuning element of tuning section 103. This effect wasminimized by setting the duty factor of light source 800 below 85% byincreasing the scan amplitude beyond the tuning range of the laser, thusensuring that the more linear portion of the sinusoidal scannermechanism was utilized.

The average optical power measured after the laser output from anembodiment of light source 800 coupled into a single mode fiber wasapproximately 15 mW. For each complete cycle of the resonant scanner,the embodiment of light source 800 produces one forward (shorter tolonger wavelengths) and one backward (longer to shorter wavelengths)sweep of its wavelength range. The forward sweep was found to be ofslightly higher output power, approximately 10%. However, when theimaging properties of the forward and backward sweeps of the laser wereindividually tested, little or no differences could be seen in theirquality. The peak output power was estimated to be about 30 mW, which isapproximately the power achieved when light source 800 is static, andlasing at the approximate peak of the ASE curve is shown in FIG. 12.Therefore, the 450 mm cavity length is short enough to allow sufficienttime for each wavelength to reach its saturation power even when thecavity is being scanned at a 16 kHz repetition rate.

As illustrated above with respect to FIG. 8, a light source 800according to the present invention includes a gain element 811positioned in a multi-mode laser cavity with a fast tuning section 103.Each component that can be utilized for these elements has certaintrade-offs to be considered. For each particular design specification,components in light section 101, transport medium 102, and tuningsection 103 are chosen carefully while considering these trade-offs.

The choice of gain element 811 is based first on the desired centerwavelength, total tuning range, and optical output power. Once materialsthat fulfill these major parameters are determined, the elements withthe greatest tuning range and highest output power are typicallyselected as the first candidates. The tuning range is indicated by thespectral plot of the ASE for a gain element under consideration. Theoptical power is determined by the optical damage threshold of gainelement 811, which is typically limited by the output facets of gainelement 811. The length of gain element 811, along with it's index ofrefraction, will determine the free spectral range (FSR) of gain element811. The FSR will be the characteristic frequency, or if converted towavelength, the characteristic wavelength, of the ripple of the laser aspreviously described. For example, if the light source is intended to beused as an OCT light source, then this ripple from gain element 811will, if it is large enough, create a ghost image. This ghost image(which is a reproduction of the main image just shifted in depth) willoccur at a depth that is related to the optical path length of gainelement 811. In the example gain element discussed in R. Huber et al., aghost image at approximately 3.55 mm (1 mm times the 3.55 index of thematerial) is formed. While this may not be a significant issue in ahighly scattering medium, such as the samples used in the Huber work,other application could suffer by presence of such a ghost image.Extending the length of gain element 811, for example to twice itsoriginal length, or 2 mm for example, could potentially shift the ghostimage out of the range of the primary image of the imaging deviceutilizing that gain element 811.

Once the gain element material and length of gain element 811 aredetermined, the reflectivity of the facets of gain element 811 can thenbe chosen. Two basic approaches have provided reasonable results. One isto select a gain element with the highest possible reflectivity for theleft facet (referring to FIG. 8, gain element 811); ideally this shouldbe in the range of 90% or higher. The laser cavity would then beconstructed with the mirror indicated as element 815 in FIG. 8 replacedwith an partial reflector so as to act as the output coupler of theresulting laser. This is often the first preliminary choice in that itallows the system to be functionally tested so that the best outputcoupler reflectivity can be determined for a particular laser design.This approach is advantageous because it is normally easier to exchangeelement 815 than it is to change the reflectivity of the facet of gainelement 811.

The second approach is to select a reflectivity of gain element 811 inthe range of 20% and design the light source such that the outputcoupler is the left facet of gain element 811. In both cases, the otherfacet of the gain element 811 is AR coated, and ideally the residualreflectivity is reduced by also utilizing a curved waveguide. Once gainelement 811 has been processed (coated using coating technology wellknown in the field of ECL production) to achieve the desiredreflectivities, the gain element is tested using an OSA to determine itsASE spectrum as well as the degree of ripple present on the spectrum. Ifthis ripple is found to have a wavelength period that matches the etalonmodes of the cavity, and if the magnitude of the ripple is less than 5%,then the gain element is assumed to be appropriate if the wavelengthrange is also acceptable. If it is between 5% to 10%, then care needs tobe taken to ensure that the rest of the cavity has fairly low losses andthat the strength of the back coupling from the extend cavity back intothe gain element is large. The actual intended application will set theupper limit on the ripple.

After the selection of gain element 811 has been made, then thecollimating optic lens system 812 in FIG. 8, can be considered. Here themain factors are the optical performance of the lens system at thedesired wavelength and over the desired wavelength range. Ideally thelens system should provide nearly diffraction limited performance;however because the light field emitted from most semiconductor gainelements is far from ideal, a broad range of optical elements can beutilized in lens system 812. There are many inexpensive molded glassaspheric lenses as well as GRIN lenses that have been tested and foundto provide acceptable results. The other concerns would be the qualityof the AR coatings and the any chromatic effects that could limit tuningrange of light source 800. The focal length of the optic is chosen sothat the collimated beam diameter provides for full illumination of thediffraction grating of tuning section 103 to provide the greatestwavelength resolution of the wavelength tuning section. The performanceof both the diffraction grating and the performance of the achromaticdoublet that will be described is effected by the beam diameter.

The transport medium 102, which can be free space of the cavity formedbetween the reflective surfaces of gain element 811 and mirror 815, canbe used to adjust the space between adjacent longitudinal cavity modes;this adjustment is made to achieve the appropriate number of cavitymodes per unit time passing through the filter function of the cavity asit tunes, as well as providing for a sufficient number of modes fallingwithin the filter function for a given configuration of the wavelengthtuning and selection section. With the overall coherence length beingroughly determined by the width of the filter function formed in thecavity, the stability of the laser, the uniformity of the coherencelength across the tuning range, the intensity noise, as well as theripple can all be effected by the choice of the overall length of thecavity. Typically a short cavity length can be achieved while ensuringthe design goals of the light source. The resolution of the wavelengthtuning and selection section was a significant factor in the ability toshorten the cavity length.

Next diffraction grating 813 is chosen. The size of grating 813 asutilized in FIG. 8 is typically limited by the mass that the high speedresonant scanner 817 can support. Because the output beam from gainelement 811 is typically elliptical, diffraction grating 813 can eitherbe chosen to have an aspect ratio that makes maximum use of theavailable mass budget for the grating, or the beam shape can be alteredwith, for example, an anamorphic prism pair. The R. Huber et al.publication did not require the additional complexity of beam shapingoptics because the beam shape and large angle of incidence on thegrating provided for good utilization of the available grating area. Ofprimary concern when picking a grating is its efficiency across thewavelength range of interest. Additionally it should be confirmed thatthe polarization of the incidence light field is aligned with thepreferred polarization of the diffraction grating, if in fact it hasone. The ruling density can be chosen so that the diffracted light fieldcan be swept across the optical axis defined by the achromatic lens andthe slit/mirror assembly. Additionally, the desire to constrain thelasing action to approximately 80% to 85% of the scan time furtherlimited the choice of ruling densities, which would ideally be made ashigh as possible so as to provide the narrowest possible filter functionfor the cavity. This filter function is further defined by the choice ofthe slit width. A wider slit generally results in a larger output powerbut would also generally result in a shortened coherence length for thelight source.

The next component after the grating in FIG. 8 is the achromatic doubletlens 814. The achromatic doublet accepts the collimated but dispersedbeam from the diffraction grating 813 and focuses a particularwavelength that is traveling along the optical axis through the slitonto the high reflector that forms the back end-mirror of the cavity oflight source 800. Since the quality of the achromat in part determinesthe shape filter function of the wavelength selection section thiselement should provide near diffraction limited performance across thewavelength range of interest. Due to the geometry, whereby only thelight that passes through the system along or very near to the opticalaxis is able to return to the gain element for amplification, this lensneeds to have very good on-axis performance over a broad wavelengthrange. This was achieved by designing the achromat using a set ofwavelengths that spanned the wavelength range of interest. Additionallythe quality of the AR coatings on the achromatic lens system 814 wasdetermined to play an important role. High quality AR coatings on bothof the lenses of achromatic lens system 814 were utilized in order toensure low losses in the external cavity.

As for the focal length of achromatic lens system 814, a range of focallengths were tested, with good performance being achieved across a rangefrom about 15 mm to about 50 mm. The shorter focal lengths were found toprovide slighter better overall performance in terms of wavelengthrange, power, and coherence length. While the light source described inHuber et. al. had a 45 mm achromatic lens system, better results arepossible using the shorter focal length optics. Choice of achromaticlens system 814 can also consider the largest diameter beam that can besupported by the cavity. Typically, the larger the diameter the betterthe performance. However, if a shorter focal length achromatic lens isused, then their small diameters need to be considered. The designchallenge here is to provide the greatest degree of wavelengthselectivity while introducing the lowest possible loss at the centerwavelength of the filter.

The next component to be considered in FIG. 8 is slit 816. This elementallows the central wavelength of the filter function formed in lightsource 800 to pass while blocking other wavelengths. From designsimulations as well as from experimentation, silts with widths in therange of 5 μm to 15 μm were found to provide the best overall results,with acceptable power and coherence length, for slit 816. When the focallength of achromatic lens system 814 was near the short end of therange, then the smaller width slits work better. Slits are commerciallyavailable from National Aperture and were laser machined in metal foiland blackened so as to minimize unwanted back reflections.

Mirror 815, to which slit 816 is bound, ideally would have substantially100% reflectivity across the wavelength range of light source 800. Thisgoal was nearly achieved by using a high quality broadband mirrorprovided by Thorlabs, Inc.

The use of the cat-eye configuration in tuning section 103, as notedabove, provides a design with good performance over a broad range ofwavelengths, while also providing a system that is relatively tolerantof misalignment as previously noted.

Other embodiments of light sources according to the present inventionare illustrated in FIGS. 1-5. As described below, a slit mirror isunderstood to mean a narrow width slit shaped reflector e.g. a mirror, atransparent slit with a mirror directly behind or a transparent slitwith a reflector behind together with an optical system which retroreflects the radiation back through the slit. Further, the term light incontext of optical radiation stands for visible, ultraviolet or infraredradiation.

FIG. 1 illustrates an embodiment of light source 100 according to thepresent invention. Light source 100 is formed from a tunable laser thatextends from a first end-reflector 11 to second end-reflector 19. Apresumed out coupling of part of the optical power is done byconventional methods, e.g., at first end-reflector 11 or at the secondend-reflector 19 or by using the reflection of the primary incidentlight or the retro reflected dispersed light from the dispersive element16 or by inserting an optical beamsplitter into the beam path within thelaser cavity of light source 100. The radiation is generated in thelight generating and optical gain medium 12. Light then passes throughbeam converting optics 13. The collimated outgoing beam from 13 enterspropagation medium 14. The radiation may be propagating freely in themedium or be bound to a channel e.g. an optical fiber. Through a properchoice of medium the optical length of the cavity and hence thelongitudinal mode distance in the laser device can be selected. Thecollimated beam a, which leaves the propagation medium 14 hitsdispersive element 16 either directly or after having been deviated by amoving reflector. Dispersive element 16 is rotated with a fastmechanical rotator 15. Light from dispersive element 16 is coupled intofocusing optics 17 onto slit mirror 19, which is positioned in the focalplane of focusing optics 17.

FIG. 2 shows another embodiment of light source 200 according to thepresent invention. As shown in FIG. 2, light from transport medium 14 isincident on a scan mirror 210 that is mounted on a fast mechanicalrotation scan arrangement 15. Light from scan mirror 210 is coupled intoachromatic lens 11, which is positioned such that scan mirror 210 is inits focal plane. Light from achromatic lens 11 is coupled intoachromatic lens 212, which has a focal length such that scan mirror 210is at the combined focal length of lenses 212 and 211. Grating 16 isplaced approximately at the focal length of lens 212 from lens 212. Thequantity a(λ₀), then, is a collimated beam of wavelength λ₀;

The dispersed beam b, which is a collimated beam of all wavelengths, inFIG. 1 and FIG. 2, is focused onto the filtering reflective slit mirror19 by lens system 17. The wavelength is scanned across the slit mirror19 by the moving dispersive element 16 in FIG. 1, respectively by themoving reflector 210 in FIG. 2.

In the embodiment shown in FIG. 1, the dispersive element 16 rotates bymeans of a fast mechanical scan arrangement 15. The dispersed radiationb is focused on the end slit mirror 19.

In the embodiment shown in FIG. 2, the incident collimated beam a ismoved angularly by the fast rotating mechanical reflector arrangement ascan reflector 210 formed of a mirror mounted on a fast mechanicalrotation scan arrangement 15 and the optical system formed by lenses 211and 212. The collimated beam a hits a stationary dispersive element 16at different incident angles, depending on the momentary rotationalposition of the mirror 210. The dispersed radiation, which is retroreflected by the end slit mirror 29, thus varies in wavelength with thefast rotation of mirror 210.

In the embodiment shown in FIG. 3, the stationary dispersive element 16directs the collimated dispersed beam b in an angle that depends on thewavelength λ. A spectrum is formed in the focal plane 18 of the focusingoptics 17. The spectrum in focal plane 18 is scanned by the slit mirrorassembly 313 fixed on a rotating wheel. Slit mirror assembly 313 is awheel with a peripheral regular pattern of slit mirrors that is placedin the focal plane 18 of focusing optics 17. The moving strip mirrorsconstitute the second end-reflectors of the optical cavity. Slit mirrorassembly 313 is driven by a drive motor 314.

In the embodiment shown in FIG. 4, a composite of the embodimentsillustrated in FIGS. 1 and 3 is used. This arrangement makes it possibleto use one and the same device for the two different scanning modes ofFIGS. 1 and 3. In the scanning mode, dispersive element 16 is scanningwhile one of slit mirrors 313 on the wheel is used as a stationary endslit mirror. In the scanning mode of another embodiment, the dispersiveelement 16 is stationary while the wheel with the slit mirror assembly313 is spinning.

In the embodiment shown in FIG. 5, the spectral-optical-filtering deviceis a slit or pinhole internal to a lens system 515 that is deposited infront of the dispersive element 16. Lens system 515 is an optical lenssystem with an internal spectral filtering slit or pinhole. In someembodiments, lens system 515 can be formed from a spectral filter in theform of a lens pair, separated by the sum of their focal lengths with anarrow slit in the range of about 5 μm to about 25 μm placed at thefocus of the first lens is used. And hence for this configuration, thespectral filtering element can be considered to be embedded within thesecond section 102. The arrangement of the dispersive element shown inFIG. 5 can be of the type known as a Littrow configuration whereas theother four preferred embodiments can use a modified Littman typeconfigurations.

The embodiments of laser system illustrated in FIGS. 1-5 all show thesame arrangement of parts for the first and second sections, with justthe third section changing with each new configuration. In someembodiments, the first optical surface 11 indicated in FIG. 1, andidentified as the “First end-reflector of the cavity,” could also be alow reflectivity output surface of the gain element with an additionallens similar to lens 37 of FIG. 3, which would act to collimate theoutput of the optical radiation from surface 11 of gain medium 12. Aseparate end-reflector can be coupled to gain element 12 to form thecavity.

In some embodiments, the moveable wavelength dispersive device 16 shownin FIG. 1 and the scan mirror 210 shown in FIG. 2 both are placed onfast angular scanning devices 15. The angular scanning device 15 maysuitably be driven by a galvanometer or similar mechanism in a resonantmode, a coil induction mechanism, a piezoelectric mechanism, anelectro-optic beam deflector, an acousto-optic beam deflector, an MEMSbased beam deflector, or an electrical motor mechanism.

In experimental tests of an embodiment of light source 100 such as thatshown in FIG. 1, a resonant scanner was used to rotate an opticalgrating. The output of the second section was chosen to be a free spacebeam that was collimated such that the collimated beam nearly fullyilluminated a diffraction grating 16 that was included in third 103section for this embodiment. The third section 103 included a resonantscanner 16, a diffraction grating 15, a lens 17, and a narrow 10 μm slitmirror 19, which is a 10 μm slit attached directly to a mirror. Onefamily of resonant scanner 16 that was successfully tested forutilization in embodiments of the present invention is the model SC-30type from Electro-Optical Products Corp. of Glendale N.Y.; the range ofscanners tested provided laser repetition rates in the range of 10 KHzto 24 KHz. Diffraction gratings 16 were mounted directly onto the movingportion of resonant scanner 16 in place of the mirrors that aretypically offered on such devices. Gratings 16 were fabricated onsubstrates that were approximately 1 mm thick, and ranged from about 6mm high by about 8 mm wide to about 5 mm high to about 6 mm wide. Theactual size was determined by the specific resonant frequency of theresonant scanner, as the scanner frequency increases the mass of thegrating was reduced. The ruling density of the gratings tested rangedfrom 800 lines per millimeter to 1100 lines per millimeter; in eachinstance the length of section two of the laser could be adjusted tomaximize the dynamic coherence length of the laser as well as the otheroperating parameters of the laser such as power, wavelength tuningrange, stability, optical output noise, and suppression of unwantedetalon effects within the gain element.

The collimated light field from section two 102 would enter sectionthree 103 and be incident upon the diffraction grating 16. The angle ofincidence was maximized so as to illuminate the maximum area of thegrating 16 and to provide the greatest angular dispersion of the lightgenerated by the broadband gain element 12 of section one 101. Thedispersed light was then incident upon by a high quality achromatic lens17, and thus the dispersed light field was then focused down onto thenarrow slit and mirror assembly 19. The lenses tested with good resultswere either high quality achromats designed for use in the wavelengthrange of the gain medium or glass aspheric lenses. All the lensesutilized high quality multilayer antireflective coatings to minimize theintra-cavity losses. The focal lengths successfully used were in therange of about 10 mm to about 50 mm for the achromatic lenses, and inthe range of about 6 mm to about 8 mm for the aspheric lenses.

The final optical component of this section, slit mirror 19, included anarrow slit cut into a thin foil that was ⅜″ in diameter. The slitwidths that were successfully tested were in the range of 5 μm to 25 μmin width and 3 mm in height. The slit was then mounted in direct contactwith a high quality mirror to form slit mirror 19. The mirror thatprovided the best results were commercially available mirrors thatfeature a broadband dielectric coating that was highly reflective acrossthe wavelength range of interest. See Thorlabs model number BB1-E04 forthe 1.3 μm wavelength range and BB1-E03 for the 800 nm wavelength range.The lens was placed approximately 10 to 20 mm from grating 16, thedistance between the lens 17 and grating 16 was not found to be veryimportant for the successful operation of the laser; however in generala slight improvement in the coherence length and output power was foundthe closer that lens 17 was placed to grating 16. With the resonantscanner 15 off, the lens 17 was placed on the optical axis of thesystem. In this case, the optical axis is defined as the path defined bythe center wavelength of the spontaneously emitted light which isemitted by the broadband gain element 12 as it propagates away from thegrating 16 in a direction defined by the grating characteristics and theangle of incidence for the incoming light field. The lens 17 thuspositioned acts to focus the light field to an area that is one focallength away. At this point the slit and mirror assembly was placed sothat it acted as both a spectral filtering element as well as theretro-reflector for the laser resonator.

With proper alignment and full electrical power to the first element,the embodiment of light source 100 described above will lase. When theresonant scanner 16 is switched on, light source 100 would begin to scanacross the wavelength range determined by the gain medium 12 and thecavity losses. When lasing, the output power was larger than 15 mW forone of the gain elements tests. The tests illustrated a tuning rangegreater than 138 nm and a coherence length (L_(c)=2×HWHM of the fringecontrast measurement within a Michelson interferometer) measured to belonger than 11 mm. A number of gain elements were tested successfully.The actual gain element chosen will depend on the desired centerwavelength, optical power, and tuning range required. Semiconductor gainelements are readily available from a number of suppliers (e.g.,InPhenix, Inc., of Livermore, Calif.).

A comparison between the embodiments illustrated in FIGS. 1 and 3results in the following observations. The optical arrangement lightsource 100 illustrated in FIG. 1 is obviously much simpler than that oflight source 200 illustrated in FIG. 2. On the other hand the size ofthe dispersive reflector 16 that can be moved rapidly is rather limited.Commercial resonant scanning mechanisms operating in the range of 5 kHzto 20 kHz are limited in the size of the optical element they cansupport. For the resonant scanners used in these experiments, thediffraction gratings utilized for dispersive element 16 wereapproximately 5 mm high, and 8 mm wide by 1 mm thick for the 5 kHzscanners and were smaller for the higher frequency scanners. Thislimited the number of grating lines that can be illuminated and hencealso limited the resolution of the grating that can be obtained. Thereis not a corresponding limitation in the embodiment shown in FIG. 2since a nearly perpendicular incident beam a can be used on a scanningreflector as opposed to the necessarily very oblique incidence on thescanning grating. The stationary grating utilized in light source 200illustrated in FIG. 2 allows for a much larger grating area than therotated grating utilized in light source 100 illustrated in FIG. 1.

The slit mirror 19 used as a filtering end mirror is an intricatecomponent to manufacture. In experiments, a slit in a metal foil incontact with an underlying mirror film has been successfully used. Thenarrower the slit is made, the better the filtering action becomes. Theintensity of the laser radiation, however, naturally goes down at thesame time. In experiments, slit mirror widths in the range 5-25 μm havebeen found optimal with respect to the balance of filtering andintensity performance; however other slit widths can be utilized.

If a slit mirror with adjustable width is advantageous for filtering, aspectrometer slit together with a retro reflecting optical system behindthe slit can be used. The optical system shall image the slit ontoitself. A planar mirror with a collimating lens or a planar mirror andtwo lenses is positioned to collimate the light from the slit and thenfocus it and then reflect it with the mirror placed at the focus of thesecond lens. Alternatively, a concave mirror may serve this purpose.Additionally, two movable non-transparent plates could be held in placeon top of or in close contact to a mirror. Two razor blades were testfor this purpose, one attached to a translation stage, and one fixed tothe surface of the mirror; this approach provided acceptable results.

In the embodiments illustrated in FIGS. 3 and 4, a rotating filteringand retro reflecting device 13 is utilized. A number of slit mirrors arepositioned with an equal separation close to the periphery on a spinningwheel; attaching thin slits machined in metal foil to a 50 mm diametermirror with the aid of an alignment fixture provided a workablesolution. A shaft was then attached to the back of the mirror and slitassembly to provide a completed retro-reflecting wheel 13. The wheel isdriven by an electric motor that functions as drive motor 14. The scantime, the time it takes for one of the slit mirrors to scan the relevantwavelength range in one direction, can not be made as short as with thefast resonant scan mechanisms that may be used in the embodiments shownin FIGS. 1 and 2. An advantage with the motor driven spinning wheel, asopposed to the faster resonant scan mechanism, is that that the scantime can be varied continuously.

The combination of the two scanning principles is illustrated in FIG. 4.A simultaneous scanning provides a powerful tool for acquiring images atslower speed for applications that would otherwise result in data setsthat are unmanageably large. Additionally, the extended coherence lengthof the slower scan speeds are of potential value when utilizing thesystem in OCT applications; inside, for example, the esophagus, wherethe distance between the tissue and the probe could vary dramaticallythe extended coherence length would allow for a larger imaging range.

In the embodiment shown in FIG. 3, the third section 103 includesstationary grating 16, a lens 17, and a rotating multi slit mirror 313.The ruling density of the gratings 16 tested ranged from 800 lines permillimeter to 1070 lines per millimeter. In each instance the length ofthe second section of the laser, i.e., the length of the cavity, couldbe adjusted to optimize the dynamic coherence length, power, stability,optical output noise, and suppression of unwanted etalon effects withinthe gain element. The collimated light field from the second section 102entered the third section 103 and was incident upon the diffractiongrating 16. The angle of incidence was maximized so as to illuminate amaximum area of the grating 16 and to provide the greatest angulardispersion of the light generated by the broadband gain element 12 ofsection one 101. The dispersed light is captured by a high quality lens17 and focused to an image plane 18. In the image plane, the light of agiven wavelength is represented by a vertical line; moving across thisimage plane different wavelengths are encountered.

The lenses that tested with good results were high quality achromatsdesigned for use in the wavelength range of the gain medium. All thelenses utilized high quality multilayer antireflective coatings tominimize the intra-cavity losses. The focal lengths successfully usedwere in the range of 30 mm to 50 mm. Single lens configurations as wellas multi lens configurations were tested; the multi lens configurationsprovided slightly better performance in term of optical power andcoherence length. The final optical component of this section wascomprised of a multi-slit mirror 313. The slit mirror had 20 reflectiveslits, each 20 μm wide. The reflective surface was protected aluminum.About 700 μm of material from either side of the 20 μm reflective slithad been removed by a dicing saw; the remaining reflective area betweenthe reflective slits was covered with black masking tape. Other kinds ofslit mirrors had been tested with good results, like several slitsmachined into thin foil disks mounted on an Al-protected mirror. Themulti slit mirror was mounted on a brushless servomotor P/NTG2385-Delta, Thingap Corp of Ventura Calif., which was driven by anAdvanced Motion Controls Brushless servo amplifier P/N B30A8. Thenominal current was 4 A current at 45 V. The power supply used was abench top BK precision 60 V 6 Amp DC power supply.

The number of slits that can be used is set by the combination of chosendiffraction grating 16, focal length of the lens 17, diameter size ofthe lens 17, the wavelength range of the gain medium 12 in the cavity,and the desired duty factor of the light source.

Lens 17 can be Thorlabs achromat lens AC254-050-C positioned 38 mm fromthe grating 16 (point on grating that beam hits to the front edge of thelens cell). The distance from the front edge of the lens cell (SM1L05)to the reflective slit was 50 mm and gave a 3 dB point of 111 nm tuningrange when the the laser current was 299.7 mA, and the motor was runningat 132 Hz. The coherence length of the resulting laser was measured tobe between 17.2 and 28.7 mm, depending on scan time.

There is a connection between scan time and coherence length of theradiation generated. In experiments with the multi-slit mirror,described above, this relation was studied. The coherence length of theradiation as a function of scan time was measured with a fiber basedMichelson interferometer. FIG. 6 shows the coherence length in mmplotted vertically and the scan time in ps horizontally. The graph shownin FIG. 6 clearly shows a linear decay of the coherence length withdecreasing scan time.

The optical systems 17 shown in FIGS. 3 and 4, respectively, should beof very good achromatic quality to give an almost perfectly flat focalarea. The same is true for lens system formed by the lenses 211 and 212and its ability to realize a planar focal area in between the twolenses.

The embodiment shown in FIG. 5 illustrates the least complicatedconfiguration of embodiments of light source according to the presentinvention and is apt to give a high output energy. In an experiment, thetwo lenses of lens system 515 utilized in an embodiment of FIG. 5 wereThorlabs C170TM-C, f=6.16 mm and the slit between the lenses was 10 μmwide. A continuous power of 22 mW was reached, and the coherence lengthwas measured to be approximately 2 to approximately 4 mm depending oncavity length. The wavelength tuning range was around 130 nm.

FIG. 7 illustrates another embodiment of compact scanning laseraccording to the present invention. In the embodiment shown in FIG. 7, ascanning mirror 15 that deflects the beam to a grating (indicated byb(λ)) that also operates as an end mirror in a Littrow configuration. Insome embodiments, a spectral filter section can also be used as seen inFIG. 7. The embodiment of light source illustrated in FIG. 7 departssignificantly from the embodiments illustrated in FIGS. 1-5 and 8 inthat there is a substantial change in the overall cavity length that isa result of using a diffraction grating in a Littrow orientation as thesecond end-reflector of the cavity. As the collimated beam is scannedacross the diffraction grating 16 by the scan mirror 75, which includesa mirror mounted on a fast mechanical rotational scan arrangement, thedistance between the center of the scan mirror and the diffractiongrating 16 changes as can be clearly seen from FIG. 7. The '355 patentutilizes a similar geometry; however, the '355 patent balances thedistance between the scan mirror and the grating, the distance betweenthe scan mirror to the front facet reflector, and the angle of incidenceonto the grating at the center wavelength in order to provide firstorder compensation for the axial mode tuning. The '355 patent providesan analysis of the system that can be used to ensure that the laser canbe adjusted and/or configured to operate in nearly a single mode acrossits tuning range, thus avoiding nearly all of the many tens of thousandsof mode hops-as they are know in the art of ECL systems-that areencountered. Operating at the single mode point requires substantialeffort, and, unless the system also has a means of removing the residualmode hops that are left after the first order correction has been made,the dynamic coherence length is erratic and not uniform across theentire tuning range of the light source. In some implementations of thedesign shown in FIG. 7, these same distances were intentionally adjustedso that a substantial number of mode hops, estimated to be manythousands or tens of thousands, are introduced as the system is tuned.This approach provided for a coherence length that was substantiallyshorter than that possible with a truly mode hop free tunable laser, asthey are know in the art; however it ensured that we were able to meetour design objective of providing a robust laser with a coherence lengthgreater than a few millimeters. In some embodiments of light source asillustrated in FIG. 7, a coherence length of up to about 4 mm wasobtained. The '355 patent has the cavity geometry carefully balancedsuch that the parabolic increase in the longitudinal mode number N has aminimum that occurs at the center wavelength of the laser. System thatare configured so that N as a function of wavelength did not passthrough a minimum within the tuning range of the system perform better.This configuration ensured that there was a significant change in N perunit change in wavelength as the laser was swept through its range ofwavelengths. Operating with geometries that provided this change in Nproved to provide a highly stable coherence length across the entiretuning range of the laser, whereas operating the same laser balanced toprovide the first order compensation as per the '355 patent did notgenerally offer this same performance. Further, a multimode systemperforms better than the single mode system disclosed in the '355patent.

In general, imaging applications such as OCT function best with a lightsource according to the present invention that has a coherence length ofgreater than about 3 mm, a tuning range in excess of 5% to 10% of thecenter wavelength, a power of about 0.5 mW or greater for lasersoperating in the 850 nm range and about 5 mW or greater for lasersoperating in the 1300 nm range, scan repetition frequencies of greaterthan about 10 kHz, and small ripple. Additionally the mode spacing, aswell as the number of modes that can lase simultaneously, needs to bebalanced such that the power fluctuations or intensity noise can besubstantially removed via a balanced detection scheme and that thecoherence length does not vary substantially as the system scans. Inlarge part the coherence length depends on the filter function of thecavity, which in turn is most easily adjusted by changing the filterfunction of the fast wavelength tuning section. The stability of thecoherence length over the tuning range can be important in someutilizations and is adjusted by changing the spacing as well as thenumber of modes that are supported by the cavity. The tuning range isdependent on the gain element, component losses, wavelength dependentcomponent losses, scanner amplitude (whether the scanner amplitude issufficient to cover a broad range of frequencies produced by the gainelement), and the diffraction grating characteristics or thecharacteristics of whatever wavelength tuning and filtering element ischosen. If the scanning element provides a sinusoidal scan, then it isuseful if the amplitude of the scanning element is broad enough to coverabout 120% of the bandwidth of the gain element. The grating design isdependent on the ruling, blazing, angle of incidence, and area ofillumination.

The cavity length can be short, medium, long, or may vary. Shortcavities have wider mode spacings and hence few modes for a fixed filterfunction; however long cavity lengths limit the scan rates because ofthe round trip time for the photons in the cavity. The power of thelaser is dependent on the optical gain element, damage thresholds of theoptical components in the cavity, the operating frequency, and cavitylosses. The spacing of the modes can be well defined and depends on theoptical path length of the cavity. The number of modes supported in thecavity depends on the filter function, loss in the cavity, gain, and thealignment of the cavity. The filter function is dependent on thegeometry of the cavity, the slit width of the slit-mirror, thediffraction grating, and the waveguide. The scan speed of the lightsource is dependent on the scanner, the diffraction grating and theangle of incidence of the light onto the grating, and the wavelengthrange of the laser.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A tunable multimode laser light source, comprising: an optical cavityformed between two reflectors, the optical cavity supporting a pluralityof optical modes; an optical gain medium positioned within the opticalcavity; and a tuning section positioned in the optical cavity, thetuning section being coupled to receive light from the gain medium andtune between modes of the plurality of optical modes, wherein anexternal-cavity wavelength-tuned, multi-mode laser is formed with acoherence length of greater than about 2 mm.
 2. The source of claim 1,wherein the tuning section is capable of tuning over a tuning range ofgreater than about 5% of a center wavelength of the optical gain mediumand capable of scanning at a wavelength scan frequency between about 20Hz and about 50 kHz.
 3. The source of claim 1, wherein the tunablemultimode laser light source has an output power of greater than about0.5 mW.
 4. The source of claim 1, wherein an average of the plurality ofmodes under a filter function of the cavity is dependent on thewavelength tuned by the tuning section.
 5. The source of claim 4,wherein the tuning section changes between sets of modes included in theplurality of modes supported by the optical cavity.
 6. The source ofclaim 1, wherein a length of the optical cavity is substantiallyconstant through the tuning range of the tuning section.
 7. The sourceof claim 6, wherein the tuning section holds the length of the opticalcavity substantially constant.
 8. The source of claim 1, wherein thelength of a portion of the cavity is adjusted to offset a change inlength introduced by the tuning section.
 9. The source of claim 1,further including an optical propagation medium positioned in the cavitybetween the optical gain medium and the tuning section.
 10. The sourceof claim 9, wherein the optical propagation medium is free space. 11.The source of claim 9, wherein the optical propagation medium is anoptically transmissive liquid, gas, or solid.
 12. The source of claim 9,wherein the optical propagation medium is an optical fiber.
 13. Thesource of claim 1, wherein the optical gain medium is a semiconductorgain element.
 14. The source of claim 1, wherein tunable wavelengthselecting section comprises a dispersive element mounted on an opticalscanner, the dispersive element receiving light from the gain medium; alens system coupled to receive light from the dispersive element; and aslit-mirror optically coupled to the lens system, the slit mirrorpositioned in the focus plane of the lens system.
 15. The source ofclaim 1, wherein the fast mechanical wavelength tuning section includesa dispersive part, a spectral optical filtering part, and a reflectorpart.
 16. The source of claim 15, wherein the reflector part is thedispersive part.
 17. The source of claim 3, wherein the reflector partserves as a second end-reflector of the cavity.
 18. The source of claim3, wherein the reflector part is combined with the spectral opticalfiltering part.
 19. The source of claim 1, wherein the fast mechanicalwavelength tuning section includes a dispersive part, a spectral opticalfiltering part, an optical imaging part, and a reflector part arrangedso that the optical imaging part and reflector part are configured in anoptical cat-eye arrangement.